Genome offers clue to functions of destructive wheat fungus

WEST LAFAYETTE, Ind. - One of the world's most destructive wheat pathogens is genetically built to evade detection before infecting its host, according to a study that mapped the genome of the fungus.

Stephen Goodwin, a Purdue and U.S. Department of Agriculture research plant pathologist, was the principal author on the effort to sequence the genome of the fungus Mycosphaerella graminicola, which causes septoria tritici blotch, a disease that greatly reduces yield and quality in wheat. Surprisingly, Goodwin said, the fungus had fewer genes related to production of enzymes that many other fungi use to penetrate and digest surfaces of plants while infecting them.

"We're guessing that the low number of enzymes is to avoid detection by plant defenses," said Goodwin, whose findings were published in the early online edition of the journal PLoS Genetics.

Enzymes often break down plant cell walls and begin removing nutrients, leading to the plant's death. M. graminicola, however, enters the plant through stomata, small pores in the surface of leaves that allow for exchange of gases and water.

Goodwin said the fungus seems to lay dormant between plant cells, avoiding detection. It later infects the plant, removing necessary nutrients and causing disease.

With the sequenced genome, scientists hope to discover which genes cause toxicity in wheat and determine ways to eliminate that toxicity or improve wheat's defenses against the fungus. Septoria tritici blotch is the No. 1 wheat pathogen in parts of Europe and is probably third in the United States, Goodwin said.

The genome also showed that M. graminicola has eight disposable chromosomes that seem to have no function. Goodwin said that plants with dispensable chromosomes have clear mechanisms for their maintenance, but no such mechanisms were obvious in the fungus.

Goodwin said the extra chromosomes were probably obtained from another species more than 10,000 years ago and have likely been retained for an important function, but it's not clear what that function is.

"That's a long time for these chromosomes to be maintained without an obvious function," he said. "They must be doing something important. Finding out what that is will be a key area for future research."

Goodwin collaborated with 57 scientists from 24 other institutions. The U.S. Department of Energy's Joint Genome Institute and Plant Research International of The Netherlands were equal partners in the research.

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